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Soil pH is a key factor in profitable crop production, but the pH number on your soil test report is only part of the story.

Soil pH is always one of the first numbers checked when a soil test report arrives, especially on fields with history of low pH. The pH number indicates the level of active soil acidity, or alkalinity. However, there is another related number right next to soil pH on the MSU soil test report. It is called the “lime index.” Other labs may refer to the same thing as “buffer pH” or “buffer index.” Lime index relates to the amount of reserve acidity in the soil and how much lime it will take to adjust pH to a desirable level.

Technically, reserve acidity is the acidity which is adsorbed on the surfaces of soil and organic matter particles. Practically, reserve acidity measurement is used at the soil testing lab to determine how much lime it should take to adjust pH in an acidic soil. If soil pH is above 6.8, then lime index is not a valuable number, and may not be listed on the report. For soil with pH under 6.8, the lime index usually falls between 70 and 60. No lime will be recommended if the lime index is above 70. As lime index decreases below 70, more lime is needed to bring soil pH up to the target level.

Soil with a greater reserve acidity (lower lime index number) has more capacity to resist change in pH. This resistance is sometimes referred to as “buffering.” Acidic sandy soils (low cation exchange) typically require less lime to bring about a 1.0 pH unit change than acidic clay soils, which have greater reserve acidity and are more highly buffered. It follows that a sandy soil and a clay or clay loam soil with identical acidic soil pH will have different lime indexes and lime recommendations.

For example, the table below represents two soils of different texture, but identical soil pH. The difference in lime index results in a very different lime requirement to raise soil pH to 6.5 within the top 9 inches of soil.

pH

Lime index

Lime req.

CEC

Texture

5.8

68

1.5

5.5

Loamy sand

5.8

66

3.3

10.5

Silt loam

For more information on soil pH, liming to raise soil pH, and various liming materials, refer to MSU Extension bulletin E-471, Lime for Michigan Soils.

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